1. Technical Field
This invention relates to fuel cell systems and, more particularly, to procedures for shutting down an operating fuel cell system.
2. Background Information
In fuel cell systems of the prior art, it is well known that, when the electrical circuit is opened and there is no longer a load across the cell, such as upon and during shut-down of the cell, the presence of air on the cathode, coupled with hydrogen fuel remaining on the anode, often cause unacceptable anode and cathode potentials, resulting in catalyst and catalyst support oxidation and corrosion and attendant cell performance degradation. It was thought that inert gas needed to be used to purge both the anode flow field and the cathode flow field immediately upon cell shut-down to passivate the anode and cathode so as to minimize or prevent such cell performance degradation. Further, the use of an inert gas purge avoided, on start-up, the possibility of the presence of a flammable mixture of hydrogen and air, which is a safety issue. While the use of 100% inert gas as the purge gas is most common in the prior art, commonly owned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100% nitrogen as the anode side purge gas, and a cathode side purging mixture comprising a very small percentage of oxygen (e.g. less than 1%) with a balance of nitrogen. Both of these patents also discuss the option of connecting a dummy electrical load across the cell during the start of purge to lower the cathode potential rapidly to between the acceptable limits of 0.3-0.7 volt.
It is desired to avoid the costs associated with storing and delivering a separate supply of inert gas to fuel cells, especially in automotive applications where compactness and low cost are critical, and where the system must be shut-down and started frequently. Therefore, safe, cost effective shut-down procedures are needed that do not cause significant performance degradation and do not require the use of a separate supply of inert gases at shut-down, during shut-down, or upon restarting the fuel cell system.
In accordance with the present invention, a fuel cell system is shut down by disconnecting the primary electricity using device (hereinafter, xe2x80x9cprimary loadxe2x80x9d), shutting off the air flow, and controlling the fuel flow into the system and the gas flow out of the system in a manner that results in the fuel cell gases coming to equilibrium across the cells, with the fuel flow shut off, at a gas composition (on a dry basis, e.g. excluding water vapor) of at least 0.0001% hydrogen, balance fuel cell inert gas, and maintaining a gas composition of at least 0.0001% hydrogen (by volume), balance fuel cell inert gas, during shut-down. Preferably, any nitrogen within the equilibrium gas composition is from air either introduced into the system directly or mixed with the fuel.
xe2x80x9cAs used herein, xe2x80x9cfuel cell inert gasesxe2x80x9d means gases that do not react wit hydrogen or oxygen or within the fuel cell, and do not otherwise harm cell performance to any significant extent, a d are, therefore, harmless to the fuel cell. Fuel cell inert gases may also include trace amounts of elements found in atmospheric air. If the fuel is pure hydrogen and the oxidant is air, the xe2x80x9cbalancexe2x80x9d fuel cell inert gas will be substantially all nitrogen, with a small amount of carbon dioxide found in atmospheric air, plus trace amounts of other elements found in atmospheric air. For purposes of this specification, carbon dioxide is considered a fuel cell inert gas since it does not react with hydrogen, oxygen, and is not otherwise harmful to the fuel cell to any significant extent.
If the fuel is a reformed hydrocarbon, the fuel entering the cell includes hydrogen, carbon dioxide, and carbon monoxide. The hydrogen concentration can vary from 30 to 80 volume percent hydrogen depending on the type of fuel processing system used in the power plant. In that case, air (i.e. essentially oxygen and nitrogen) is sometimes injected into the fuel upstream of the anode flow field to oxidize the carbon monoxide. The carbon monoxide is not a fuel cell inert gas, and needs to be completely converted to carbon dioxide by reaction with oxygen during the shut down procedure. Therefore, in accordance with the present invention, when the fuel cell is operated on a reformed hydrocarbon, the xe2x80x9cbalance fuel cell inert gasesxe2x80x9d may include a larger amount of carbon dioxide than in the case of cells using pure hydrogen as the fuel; however, the objective of an equilibrium gas composition of at least 0.0001% hydrogen, balance fuel cell inert gases, is the same.
It was discovered, through a series of start-up/shut-down tests, that generating an equilibrium gas composition of at least a dilute concentration of hydrogen, balance fuel cell inert gases, within the anode and cathode flow fields upon shut-down, and then maintaining at least a dilute concentration of hydrogen, balance fuel cell inert gases, within the anode and cathode flow fields during shut-down, virtually eliminates performance losses that were observed when using other shut-down procedures. It was also observed that the shut-down procedure of the present invention was able to regenerate cell performance lost by a fuel cell system that had experienced a series of shut-downs and start-ups that maintained 100% air on both sides of the cell throughout the period of shut-down. Such regeneration was surprising, since it was believed the lost performance had been due solely to catalyst and catalyst support corrosion, which cannot be reversed. This performance recovery led to the conclusion that some other mechanism was causing performance loss, and the present invention was able to reverse most, if not substantially all of that loss. The improvement is most dramatic at high current densities.
It is theorized that the additional performance decay mechanism is the formation of carbon oxides on the surface of the carbon support material and the formation of platinum oxides on the surface of the catalyst. It is also theorized these oxides form if the electrodes are subjected to a high air potential during the shut-down process, including while the cell remains idle. The surface oxides increase the wetability of the carbon and platinum causing partial flooding and, therefore, loss of performance. Factors that may be at work in the procedure of the present invention to eliminate the performance decay are the maintenance of a low electrode potential (versus the standard hydrogen electrode) during shut-down and chemical and/or electrochemical reactions involving the presence of hydrogen.
In the procedure of the present invention, the equilibrium hydrogen concentration required to be maintained during shut-down is based upon several factors. One factor is that 0.0001% hydrogen is the minimum amount needed to reduce (and maintain) the electrode potentials to less than 0.2 volts above the potential of a standard hydrogen reference electrode. At less than 0.2 volts, platinum and platinum support corrosion and carbon and platinum oxidation are virtually eliminated. Actually, hydrogen concentration of at least 1% is preferred for two reasons: first, it will reduce the electrode potential to less than 0.1 volts, at which level virtually no corrosion and surface oxidation occurs; and, second, it is easier to measure, monitor, and control than much smaller concentrations, such as 0.1% or less.
The upper end of the range for hydrogen concentration is not critical to the prevention of cell performance loss. Having 100% hydrogen throughout the cells would work fine, but is difficult and costly. For that reason, a 10% hydrogen concentration (balance fuel cell inert gases) is a more practical upper limit. On the other hand, for safety, it is preferred to have and to maintain a hydrogen concentration of less than 4%, since more than 4% hydrogen in air is considered in excess of the flammability limit. If there were less than 4% hydrogen, then any air that leaks into or is otherwise introduced into the cell would not be hazardous. If the shut-down equilibrium hydrogen concentration is maintained below 4%, the present invention will have the added benefit of allowing rapid start-up of the fuel cell by simply turning on the fuel flow and the air flow, without the necessity of first purging the hydrogen from the cathode flow field with an inert gas, such as nitrogen. For an extra margin of safety, a hydrogen concentration during shut-down of no more than about 3% is preferred.
In one embodiment of the present invention, after disconnecting the primary load and shutting off the air supply to the cathode flow field, fresh fuel continues to be fed to the anode flow field until the remaining oxidant is completely consumed. This oxidant consumption is preferably aided by having a small auxiliary load applied across the cell, which also quickly drives down the electrode potentials. Once all the oxidant is consumed the fuel feed is stopped, the fuel exit valve is shut, and air is introduced into the anode flow field (if needed) until the hydrogen concentration in the anode flow field is reduced to a selected intermediate concentration level, above the desired final concentration level. Air flow into the anode flow field is then halted, and the fuel cell gases are allowed to come to equilibrium, which will occur through diffusion of gases across the electrolyte and chemical and electrochemical reaction between the hydrogen and the added oxygen. The intermediate hydrogen concentration level is selected based upon the relative volumes of the anode and cathode flow fields, such that the resulting hydrogen concentration at equilibrium (i.e. after all the oxygen has been consumed and the hydrogen and fuel cell inert gases are fully dispersed throughout the cell) will be within the desired range. Thereafter, during continued shut-down, the hydrogen concentration is monitored; and hydrogen is added, as and if necessary, to maintain the desired hydrogen concentration level. This latter step of adding hydrogen is likely to be required due to leakage or diffusion of air into the system and/or leakage or diffusion of hydrogen out of the system, such as through seals. As air leaks into the system, hydrogen reacts with the oxygen in the air and is consumed. That hydrogen needs to be replaced, from time to time, to maintain the hydrogen concentration within the desired range.
In another embodiment of the shut-down procedure of the present invention which uses either pure hydrogen as the fuel, or a reformate with a relatively high hydrogen concentration, the primary load is disconnected and both the hydrogen flow to the anode flow field and the fresh air flow into and through the cathode flow field are shut off. This essentially traps an initial amount of hydrogen within the anode flow field and an initial amount of air within the cathode flow field. In all practically sized fuel cell systems using pure hydrogen as the fuel the trapped amount of hydrogen will be considerably more than required to consume all the trapped amount of oxygen, leaving a hydrogen concentration above the desired final equilibrium concentration. That will also be the case for reformates with high hydrogen concentrations. (An auxiliary load may also be used in this embodiment to quickly drive down the electrode potentials and rapidly consume the oxygen.)
In either case, a restricted flow of oxygen (most preferably in the form of air), beyond the initial amount, is provided directly into the anode flow field to bring on a further reduction in the concentration of hydrogen, until the gases reach an equilibrium gas composition having a desired hydrogen concentration (balance fuel cell inert gases), or a hydrogen concentration within a pre-selected range, for example, between 1% and 3%, (balance fuel cell inert gases). When the equilibrium hydrogen concentration is as desired, no further air is fed to the anode flow field. As in the embodiment first described above, the hydrogen concentration within the anode flow field is monitored during shut-down. Additional hydrogen is added, as and if necessary, to replace any hydrogen lost through leakage or through reaction with any oxygen that may leak into the system. In that manner, the gas composition is maintained within the desired range until the fuel cell system is to be started again.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.